U.S. patent number 11,358,883 [Application Number 16/268,154] was granted by the patent office on 2022-06-14 for system and method for using ultramicroporous carbon for the selective removal of nitrate with capacitive deionization.
This patent grant is currently assigned to Lawrence Livermore National Security, LLC. The grantee listed for this patent is Lawrence Livermore National Security, LLC. Invention is credited to Patrick Campbell, Steven Hawks, Maira Ceron Hernandez, Colin Loeb, Tuan Anh Pham, Michael Stadermann.
United States Patent |
11,358,883 |
Campbell , et al. |
June 14, 2022 |
System and method for using ultramicroporous carbon for the
selective removal of nitrate with capacitive deionization
Abstract
The present disclosure relates to a flow through electrode,
capacitive deionization (FTE-CDI) system which is able to adsorb
nitrates from water being treated using the system. The system
makes use of a pair of electrodes arranged generally parallel to
one another, with a water permeable dielectric sandwiched between
the electrodes. The electrodes receive a direct current voltage
from an electrical circuit. At least one of the electrodes is
formed from a carbon material having a hierarchical pore size
distribution which includes a first plurality of pores having a
width of no more than about 1 nm, and a second plurality of
micro-sized pores. The micron-sized pores enable a flow of water to
be pushed through the electrodes while the first plurality of pores
form adsorption sites for nitrate molecules carried in the water
flowing through the electrodes.
Inventors: |
Campbell; Patrick (Oakland,
CA), Hernandez; Maira Ceron (Brentwood, CA), Hawks;
Steven (Livermore, CA), Loeb; Colin (Fairfield, CA),
Pham; Tuan Anh (Livermore, CA), Stadermann; Michael
(Pleasanton, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Lawrence Livermore National Security, LLC |
Livermore |
CA |
US |
|
|
Assignee: |
Lawrence Livermore National
Security, LLC (Livermore, CA)
|
Family
ID: |
1000006368278 |
Appl.
No.: |
16/268,154 |
Filed: |
February 5, 2019 |
Prior Publication Data
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|
|
|
Document
Identifier |
Publication Date |
|
US 20200247693 A1 |
Aug 6, 2020 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C02F
1/4691 (20130101); C01B 32/336 (20170801); C01B
32/318 (20170801); C02F 2101/163 (20130101); C01P
2006/40 (20130101); C01P 2004/02 (20130101) |
Current International
Class: |
C02F
1/469 (20060101); C01B 32/318 (20170101); C01B
32/336 (20170101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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100973669 |
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Aug 2010 |
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KR |
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WO-0166217 |
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Sep 2001 |
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WO |
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WO-0190443 |
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Nov 2001 |
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WO |
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WO-2010014615 |
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Feb 2010 |
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WO |
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WO-2012148709 |
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Jan 2013 |
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WO |
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|
Primary Examiner: Jain; Salil
Attorney, Agent or Firm: Harness, Dickey & Pierce,
P.L.C.
Government Interests
STATEMENT OF GOVERNMENT RIGHTS
The United States Government has rights in this invention pursuant
to Contract No. DE-AC52-07NA27344 between the U.S. Department of
Energy and Lawrence Livermore National Security, LLC, for the
operation of Lawrence Livermore National Laboratory.
Claims
What is claimed is:
1. An ultramicroporous electrode for use in a flow through,
capacitive deionization (FTE-CDI) system for adsorbing nitrate ions
contained in water being fed into the electrode for treatment, the
electrode comprising: a carbon aerogel member having a hierarchical
pore size distribution, the hierarchical pore size distribution
including: a first plurality of ultramicropores each having a shape
forming a slit, and substantially all of the first plurality of
ultramicropores being below 1 nm in width and forming a slit-like
shape; and a second plurality of pores distributed throughout the
thickness of the carbon aerogel member and being sufficiently large
to enable fluid flow paths to be formed through the carbon aerogel
member, which enable a flow of water to be pushed through the
carbon aerogel member; the first plurality of ultramicropores
configured to selectively capture nitrate ions carried in the water
flowing through the carbon aerogel member; and wherein the first
plurality of ultramicropores collectively provide a total pore
volume of approximately 0.3 cm.sup.3/g and collectively provide a
total pore area of about 1084 m.sup.2/g when the first plurality of
ultramicropores are substantially all below 1 nm in width.
2. The ultramicroporous electrode of claim 1, wherein the first
plurality of ultramicropores is distributed throughout the
thickness of the carbon aerogel member.
3. The ultramicroporous electrode of claim 1, wherein the second
plurality of pores each comprise micron-sized pores.
4. The ultramicroporous electrode of claim 3, wherein the second
plurality of micron-sized pores are distributed throughout the
thickness of the carbon aerogel member.
5. The ultramicroporous electrode of claim 4, wherein the second
plurality micron-sized pores enable the flow of water to be pushed
through the entire thickness of the carbon aerogel.
6. The ultramicroporous electrode of claim 1, wherein the first
plurality of ultramicropores each form an adsorption site for
capturing the nitrate ions.
7. An ultramicroporous electrode for use in a flow through,
capacitive deionization (FTE-CDI) system for adsorbing nitrate ions
contained in water being fed into the electrode for treatment, the
electrode comprising: a carbon aerogel member having a hierarchical
pore size distribution, the hierarchical pore size distribution
including: a first plurality of ultramicropores each shaped as a
slit and each having a width configured to selectively capture a
nitrate ion, and the slits being distributed throughout a thickness
of the carbon aerogel member, and each one of the first plurality
of ultramicropores having a width of no more than about 1 nm, and
wherein the first plurality of ultramicropores collectively provide
a total pore volume of 0.3 cm.sup.3/gram and collectively provide a
total pore area of 1084 m.sup.2/g; and a second plurality of pores
distributed throughout the thickness of the carbon aerogel member,
and being sufficiently large to enable fluid flow paths to be
formed through the entire thickness of the carbon aerogel member,
which enable a flow of water to be pushed through the thickness of
the carbon aerogel member; and the first plurality of
ultramicropores operating to form adsorption sites to selectively
capture nitrate ions carried in the water flowing through the
carbon aerogel member.
8. The ultramicroporous electrode of claim 7, wherein the first
plurality of ultramicropores are distributed throughout the carbon
aerogel member.
9. The ultramicroporous electrode of claim 7, wherein the second
plurality of pores comprise pores on the order of microns in
size.
10. The ultramicroporous electrode of claim 9, wherein the
micron-sized pores are distributed throughout the thickness of the
carbon aerogel member.
Description
FIELD
The present disclosure relates to capacitive desalination systems
and methods, and more particularly to systems and methods for flow
through electrode, capacitive deionization (FTE-CDI) which
incorporate a new electrode construction for effectively removing
nitrate from a mixture of ions in fluid (water) flowing through the
system.
BACKGROUND
The statements in this section merely provide background
information related to the present disclosure and may not
constitute prior art.
The current state-of-the-art for water desalination is reverse
osmosis (RO). Reverse osmosis uses membranes that allow water, but
not salt, to pass through the membranes. Pressure is applied to the
feed side, pushing water across the membrane to overcome membrane
resistance, as well as the osmotic pressure. Energy use in RO
scales with the amount of water produced. For seawater, the energy
efficiency of RO is unsurpassed, however at low salt concentration
the energy efficiency of RO is significantly reduced. Furthermore,
RO membranes are non-selective, which means that one must remove
all ions to remove a particular contaminant. This further reduces
the possible efficiency of using RO to treat water for specific
trace contaminants.
Capacitive deionization (CDI) is a more recently developed
technology. Unlike membrane-based methods, CDI removes salt with
electric fields. The charged salt ions are attracted to the charged
porous electrodes and thus removed from the water. The device is
operated by applying a voltage to the two spaced apart electrodes,
which act like plates of a supercapacitor. While water passes
through the device, salt ions are attracted to the charged surface
and thus removed from the feed water. The energy cost of CDI is
proportional to the amount of salt removed, thus giving it the
potential to be more energy efficient than RO in low salinity
regimes. Because CDI is an inherently low-pressure operation and
cell and electrode components are made from low-cost materials, the
capital costs are also expected to be significantly less than
RO.
Flow-through electrode capacitive deionization (FTE-CDI) is a
technology that involves flowing feed water to be desalinated
through the porous electrodes of a capacitive deionization system,
rather than between the electrodes as in a conventional CDI device.
The assignee of the present application is a leader in the
development of this technology, as will be appreciated from the
disclosure of U.S. Patent Publication No. 2012/0273359 A1,
published Nov. 1, 2012, the disclosure of which is hereby
incorporated by reference into the present disclosure. In view of
the known advantages of an FTE-CDI system, significant interest
exists in even further enhancing and improving the capabilities of
such a system to even more effectively and efficiently perform
desalination on salt water and/or to remove other types of ions
from water.
In addition to general salinity reduction, a particular area of
interest in CDI research is the selective removal of specific ionic
contaminants for increased energy efficiency and to more
effectively utilize removal capacity. One of the major contaminants
of interest in present day CDI research is nitrate, which is
regulated by the US Environmental Protection Agency to a maximum
contaminant level in drinking water of 10 mg/L (as N) or 0.7 mM as
NO.sub.3. The concentration of nitrate in groundwater is increasing
by a reported 1-3 mg/L/yr due to a number of factors including
human activities involving agriculture, for example from fertilizer
runoff and disposal of municipal effluents by sludge spreading on
fields. Other factors contributing to the increased concentration
of nitrate found in groundwater include atmospheric emissions from
energy production sources, as well as combustion engines of present
day motor vehicles. Accordingly, there is a growing interest in
developing systems for more effectively removing nitrates, in
particular, from groundwater, making the development of effective
treatment methods increasingly important.
SUMMARY
In one aspect the present disclosure relates to a flow through
electrode, capacitive deionization (FTE-CDI) system. The system may
comprise a pair of electrodes arranged generally parallel to one
another; a water permeable dielectric arranged between the
electrodes so as to be sandwiched between the electrodes; and an
electronic circuit for applying a direct current voltage across the
electrodes. At least one of the electrodes may be formed from a
carbon material having a hierarchical pore size distribution, the
hierarchical pore size distribution including a first plurality of
nano-sized pores having a width of no more than about 1 nm, and a
second plurality of pores having micron-sized pores that enable a
flow of water to be pushed through the electrode. The first
plurality of pores form adsorption sites for nitrate molecules
carried in the water flowing through the at least one
electrode.
In another aspect the present disclosure relates to an
ultramicroporous electrode for use in a flow through, capacitive
deionization (FTE-CDI) system for adsorbing nitrate molecules
contained in water being fed into the electrode for treatment. The
electrode may comprise a carbon aerogel member having a
hierarchical pore size distribution. The hierarchical pore size
distribution may include a first plurality of ultramicropores
randomly distributed throughout a thickness of the carbon aerogel
member, and each forming a slit having a width of no more than
about 1 nm; and a second plurality of micron-sized pores randomly
distributed throughout the thickness of the carbon aerogel member.
The micron-sized pores are sufficiently large to enable fluid flow
paths to be formed through the entire thickness of the carbon
aerogel member, which enable a flow of water to be pushed through
the thickness of the carbon aerogel member. The first plurality of
pores form adsorption sites for capturing nitrate molecules carried
in the water flowing through the carbon aerogel member.
In still another aspect the present disclosure relates to a method
for making a carbon aerogel electrode material. The method may
comprise making a wet organic sol-gel form; carbonizing the sol-gel
form at a temperature of from about 900.degree. C. to about
1000.degree. C., for from about 2 to about 4 hours; and activating
the carbonized sol-gel under carbon dioxide flow, for from about
0.5 hours to about 1.5 hours, at from about 900.degree. C. to about
1000.degree. C.
Further areas of applicability will become apparent from the
description provided herein. It should be understood that the
description and specific examples are intended for purposes of
illustration only and are not intended to limit the scope of the
present disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawings described herein are for illustration purposes only
and are not intended to limit the scope of the present disclosure
in any way.
FIG. 1 is a high level block diagram of one embodiment of a FTE-CDI
system in accordance with the present disclosure for removing
nitrate from a flowing fluid, for example from flowing water
through the system
FIG. 1a is a highly simplified enlargement of a portion of one of
the electrodes shown in FIG. 1 illustrating in a highly simplified
manner the microporous (i.e., nanometer scale) pore structure and
the micron sized flow paths formed in each of the electrodes, the
microporous structure being well suited for capturing nitrate
contained in a flowing fluid;
FIG. 2 is a simplified high level illustration showing a portion of
one of the electrodes in FIG. 1 to illustrate how the nitrate
molecule fits within one of the ultramicropores, while a chloride
molecule and sulfate molecule do not fit stably in the
ultramicropore;
FIGS. 3a, 3b, 3a1 and 3b1 are illustrations showing the
distribution of solvating water molecules around the disc-like
nitrate molecule, and wherein FIG. 3a shows the XY (or equatorial)
plane of nitrate, which has strongly bound water, indicated by red
in the plot of FIG. 3a1, and wherein, FIG. 3b shows the ZX (or
axial) plane, which has much weaker interactions with solvating
water, indicated by the lack of red in the plot of FIG. 3b1, which
implies that the overall shape of the solvated nitrate molecule is
disc-like;
FIG. 4 is a high level flowchart summarizing various operations
that may be performed in forming the electrodes shown in FIG.
1;
FIGS. 4a and 4b illustrate high and low magnification micrographs,
respectively, obtained from a scanning electron microscope showing
the electrodes after carbonization and activation;
FIG. 5 is a graph showing measurements of the micro-pore size
distribution from N.sub.2 adsorption measurements of the
micro-pores formed in a carbon aerogel electrode of the present
disclosure, and more particularly showing micro-pore size
distribution as a function of slit pore width;
FIG. 6 is a graph created using N.sub.2 adsorption measurements of
the micro-pores of the carbon aerogel of the present disclosure,
showing cumulative micro-pore volume as a function of slit pore
width;
FIG. 7 shows a graph of electrosorption of the carbon aerogel of
the present disclosure illustrating the concentrate collected at
different charge voltages applied to the cell, for each of ion
species NO.sub.3, CI and SO.sub.4; and
FIG. 8 is a graph showing calculated nitrate/chloride and
nitrate/sulfate selectivities from the results of FIG. 7.
DETAILED DESCRIPTION
The following description is merely exemplary in nature and is not
intended to limit the present disclosure, application, or uses. It
should be understood that throughout the drawings, corresponding
reference numerals indicate like or corresponding parts and
features.
The present disclosure relates to FTE-CDI systems and methods which
employ a new material in a new geometry to further increase the
process rate compared to the typical flow between the electrodes of
a CDI system. The present system and method introduces a FTE-CDI
system which uses a new carbon material (CO.sub.2 activated AARF)
for the electrodes of the system, described more fully in the
following paragraphs, which has a hierarchical pore size
distribution. The hierarchical pore size distribution includes a
first plurality of sub-nanometer scale pores ("ultramicropores") to
provide adsorption sites, while a second plurality of pores are
included which form micron-sized pores through which water can be
pushed at relevant flow rates without requiring a substantial
amount of energy. This material can now be used in a different
geometry: rather than passing water between the electrodes, the
water is pushed through the electrodes. Instead of relying on
diffusion, the salt is actively pushed into and out of the
capacitor, which reduces desalination time substantially.
FIG. 1 illustrates one example of an embodiment of flow-through
electrode capacitive deionization (FTE-CDI) system 100 according to
the present disclosure. The FTE-CDI system 100 may be viewed as
forming a single "cell" and includes a pair of electrodes 102 and
104, and an electric circuit 110 that energizes the electrodes 102
and 104. The electrode 102 contacts and electrically connects the
current collector 138, which electrically connects the electrode
102 to one side of the electric circuit 110, which applies a
voltage across the electrode 102. Similarly, the electrode 104
contacts and electrically connects the current collector 140, which
electrically connects to the other side of the electric circuit
110. The current collectors 138 and 140 may be made of any suitable
metals or metal alloys, but in one preferred implementation are
made from titanium. The current collectors 138 and 140 may be foils
or wires, and they may be connected to the electrodes 102 and 104
using conductive epoxy (e.g., silver epoxy) or paint (e.g., nickel
paint), which is completely sealed or potted in epoxy to prevent
corrosion. The electric circuit 110 applies a DC voltage across the
electrodes 102 and 104, which produces an electrical field between
the electrodes 102 and 104.
The electrodes 102 and 104 are arranged such that a flow of the
feed water flows through the electrodes 102 and 104 and in a
direction parallel to an electric field applied across the
electrodes 102 and 104. While only a single pair of electrodes
102/104 is shown in FIG. 1, in practice it is anticipated that in
commercial applications, the system 100 may include two or more
pairs of electrodes. Also, it is anticipated that a commercial
application of the system 100 will likely involve using a much
larger plurality of instances of the system 100, possibly
incorporating hundreds or more such pairs of electrodes 102/104,
depending on device size, salt removal, and throughput
considerations, and the intended application.
With further reference to FIG. 1, a water-permeable separator 114
made of an insulative material (e.g., dielectric material) may be
disposed between the electrodes 102 and 104 to prevent electrical
short-circuits between the electrodes 102 and 104. The separator
114 may be made of, e.g., electrolyte permeable paper or a polymer
membrane (or polymer membranes). In one implementation the
thickness of the separator 114 may be, for example, less than about
20% of an overall thickness of each of the electrodes 102 and 104,
and in one specific implementation may be on the order of no more
than about 100 microns thick, and in one particularly preferred
implementation the separator is about 20-50 .mu.m thick.
Header plates 150 and 152 are disposed to sandwich the electrodes
102 and 104 and the separator 114. The header plates 150 and 152
are made of, e.g., ultraviolet (UV) transparent acrylic material.
Alternative to acrylic, other transparent plastic materials may
also be used (e.g., polycarbonate). The header plates 150 provides
structural support to the electrodes 102 and 104 and the separator
114.
An epoxy 154 may be disposed between the header plates 150 and 152
and surrounding the electrodes 102 and 104 and the separator 114.
The epoxy 154 may be, e.g., UV-curable epoxy. The header plates 150
and 152 and the epoxy 154 define a space that accommodates the
electrodes 102 and 104 and the separator 114. In some embodiments,
a combination of the header plates 150 and 152, the electrodes 102
and 104, the separator 114, the current collectors 138 and 140, and
the epoxy 154 is referred to as a cell (e.g., an FTE-CDI cell, or a
flow through cell). Again, it will be understood that in a
commercial application, a large plurality of instances of the
system 100 (with the system representing one "cell") is likely to
be used.
The FTE-CDI system 100 includes an input flow line 122 and an
output flow line 124. In some embodiments, the input flow line 122
and the output flow line 124 are part of the system 100. In some
embodiments, the system 100 may include multiple input flow lines
and/or multiple output flow lines.
The header plate 150 includes one or more flow channels formed
therein. For example, as shown in FIG. 1, the header plate 150 is
shown to include a channel 151, although in practice it will be
appreciated that a plurality of flow channels 151 may preferably be
formed in the header plate 150 to distribute a fluid flow evenly
through the header plate. The channel 151 of the header plate 150
is in fluidic communication with the input flow line 122 and the
electrode 102. Similarly, the header plate 152 defines one or more
channels 153 therein, and more preferably, a plurality of spaced
apart flow channels. The channel 153 of the header plate 152 is in
fluidic communication with the output flow line 124 and the
electrode 104.
Because the space accommodating the electrodes 102 and 104 and the
separator 104 is sealed by the epoxy 154, water can only flow into
and out of the cell through the flow lines 122 and 124. Thus,
during operation, water flow into the FTE-CDI system 100 through
the input flow line 122, the channel 151 of the header plate 150
(or multiple channels), the electrode 102, the separator 114, the
electrode 104, the channel 153 of the header plate 152 (or multiple
channels), and the output flow line 124.
In operation during a charging stage, as the water flows through
the electrodes 102 and 104, ions from the water are attracted to
the electrodes 102 and 104 and adsorb to the surfaces of the porous
electrodes 102 and 104. During a discharging stage, to avoid ion
saturation on the electrodes 102 and 104, the electrodes 102 and
104 are short-circuited or applied with a reverse electrical
potential difference (e.g., by the electric circuit 110). As a
result, ions previously adsorbed on the electrode surfaces are
flushed into waste water flowing through the electrodes 102 and
104.
Electrode Construction
The electrodes 102 and 104 of the system 100 are new and
effectively work to capture nitrate molecules from fluids (e.g.,
water) flowing through the electrodes. As shown in highly
simplified representative form in FIG. 1a, each electrode 102 and
104 forms an ultramicroporous electrode (e.g., carbon aerogel). The
terms "ultramicroporous" and "ultramicropores", as used herein,
mean a quantity of pores which are all below, or substantially all
below, about 1 nm in width. These ultramicropores are designated by
reference number 102a in FIG. 1a, and are distributed randomly
throughout each electrode 102 and 104. In addition, micron-sized
flow paths, indicated by reference number 102b in FIG. 1a for the
electrode 102, are formed with a random distribution throughout the
electrode 102, and extend through the entire cross section of the
electrode 102. With brief reference to FIGS. 4a and 4b, the
micron-sized pores 102b form a ligament structure throughout the
thickness of the electrode 102. The ultramicropores 102a are
present on the ligaments, so they are essentially uniformly
distributed through the thickness of the electrode 102. Electrode
104 may be formed in an identical manner to also include both the
ultramicropores 102a and the micron-sized pores 102b. Only the
micron-sized pores allow flow through the thickness of the
electrode 102.
The ultramicropores 102a of the electrodes 102 and 104 are a highly
important feature which enables the system 100 to selectively
remove nitrate over other ions, especially common divalent species.
The reason for this is that the ultramicropores 102a formed in the
electrodes 102/104 (e.g., carbon aerogel) tend to have slit-shaped
pores, as shown in highly simplified form in FIG. 2, and that
nitrate is a weakly solvated disk-like ion (FIGS. 3a and 3b),
making it the perfect lock-and-key situation for selective
adsorption (FIG. 2). FIGS. 3a and 3b illustrate the nitrate ion,
with FIG. 3b illustrating particularly well the disc-like shape
that this ion has. The disc-like shape enables the nitrate ion to
be easily captured in the slit-shaped ultramicropores 102a. FIGS.
3a1 and 3b2 show graphs which illustrate the projection
distribution of water oxygens on the planes parallel (FIG. 3a1) and
perpendicular (FIG. 3b1) to the plane of the anion. In the graphs
of FIGS. 3a1 and 3b1, red=oxygen, white/grey=hydrogen and
blue=nitrogen.
The electrodes 102 and 104 were formed in the same manner, and
therefore the following discussion will reference only the forming
of the electrode 102. The flowchart 200 of FIG. 4 summarizes
various the operations used to make the ultramicroporous
hierarchical carbon aerogel monoliths ("pHCAMs") that are used as
the electrodes 102 and 104. Initially at operation 202 a quantity
of 430.5 g of resorcinol (3.92 mol, 99% Sigma Aldrich) was
dissolved in 525.0 g of DI water. At operation 204 a quantity of
626.5 g of 37% formaldehyde solution (7.84 mol, ACS grade, contains
10% MeOH, Sigma Aldrich) was then added. At operation 206 a
quantity of 15.4 g of glacial acetic acid (0.245 mol, 99+% Sigma
Aldrich) was then also added. At operation 208 the reagents were
mixed for 30 min at 40.degree. C. At operation 210 the mixture was
then poured into a Teflon mold and cured at 23.degree. C. for 46
hours, followed at operation 212 by aging for 24 hours at
70.degree. C. The aged RF blocks were then removed from the mold
and sliced into thin sheets having a thickness of from about 300
.mu.m to about 700 .mu.m, and in one instance a thickness of about
500 .mu.m. The sheets were formed by slicing the aged RF blocks
with a suitable implement, for example a band saw (e.g., Delta
Model 28-185), as indicated at operation 214. The wet organic
aerogel sheets were washed with DI water and subsequently exchanged
for acetone, as indicated at operation 216. Wet aerogel sheets were
then sandwiched between porous silicon carbide sheets, as indicated
at operation 218, and then loaded into a custom-made drying chamber
equipped with an airflow control unit, as indicated at operation
220. After loading, the drying chamber was sealed, and the air flow
rate set to 80 mL/min, as indicated at operation 222, to dry the
aerogel sheets. Dry carbon aerogel were carbonized at about
900.degree. C. to about 1000.degree. C., for about 2 to 4 hours,
and in one instance at about 950.degree. C. for 3 hours under
N.sub.2, as indicated at operation 224, and subsequently activated
for about 0.5-1.5 hours at a temperature of about 900.degree. C. to
about 1000.degree. C., and in one instance for 1 hour at
950.degree. C., with CO.sub.2 flow at 2 L/min in a 6 inch tube
furnace, as indicated at operation 226. At operation 228 the
activated carbon aerogel was then removed from the furnace and the
process ends. FIGS. 4a and 4b illustrate high and low magnification
micrographs, respectively, obtained from a scanning electron
microscope showing a portion of the ultramicroporous electrode 102
after carbonization and activation.
It is important to note that the resulting aerogel is activated to
have the .about.0.3 cm.sup.3/g microporosity with pore sizes almost
all being below 1 nm in width. FIG. 5 is a graph 300 showing
measurements of the micro-pore size distribution from N.sub.2
adsorption measurements of the micro-pores formed in a carbon
aerogel electrode of the present disclosure, and more particularly
showing micro-pore size distribution as a function of slit pore
width. FIG. 6 shows a graph 400 illustrating cumulative micro-pore
area as a function of slit pore width. It is this micro-pore size
distribution that is highly important to making the electrodes 102
and 104 selective for nitrate. Nitrate, being a planar, weakly
solvated ion, is ideal for fitting into the narrow ultramicropores
102a where other ions either cannot fit or are less energetically
stable within. This pore size distribution is extremely effective
in adsorbing nitrate over both a divalent species and a common
interferant ion like chloride.
The electrosorption selectivity of the activated aerogel electrode
102 described above was measured in a flow-through electrode CDI
cell, the results of which are shown in FIGS. 7 and 8. To do so, a
3.33 mM/3.33 mM/1.67 mM NaCl/NaNO.sub.3/Na.sub.2SO.sub.4 feed
solution were used. The CDI cell was charged at various constant
voltages (0.4-1 V) under a constant flow rate (3 ml/min) while
monitoring the effluent conductivity. After charging the CDI cell
at constant voltage and flow for an extended period of time (>25
min), the CDI cell was discharged to zero volts, and then the
resulting concentrate was collected, stopping once the CDI cell
current density decayed to a low value (0.045 mA/cm.sup.2). With
the concentrate solution in hand, it was then possible to measure
the ion concentration ratios that were adsorbed onto the electrodes
during the charging phase with ion chromatography. FIG. 7 presents
a graph 500 illustrating the resulting raw concentration values of
nitrate, chloride, and sulfate in the collected concentrate
solution. By far, the dominant adsorbed species was nitrate,
followed by chloride, and then lastly by sulfate
(NO.sub.3.sup.->Cl.sup.->>SO.sub.4.sup.2-). The sulfate
concentration scarcely deviated from the feed (FIG. 7), indicating
that it was essentially not adsorbed. These results indicate that
ultramicroporous carbon can be used as a highly selective sorbent
for nitrate and perhaps other weakly solvated planar ions, even in
the presence of divalent ions.
FIG. 8 shows a graph 600 which illustrates that the observed
nitrate selectivities are exceptionally high when compared to other
CDI research, especially given that a divalent ion (sulfate) is
present in addition to chloride. Most importantly, these results
are obtained without the need for specialized functionalization,
membranes, or coatings. Still further, these results are produced
using a relevant mixture with multivalent and chloride
interferants. This shows that the carbon aerogel electrodes 102 and
104 described herein are ideally suited for selectively removing
nitrate from ion mixtures due to an excellent match between pore
structure (narrow slits, mostly below 1 nm in width) and ion
solvation properties (i.e., nitrate is a weakly solvated in the
axial direction). The present disclosure also shows that a
particular carbon electrode microporosity can be a highly effective
way to achieve excellent electrosorptive selectivity.
While various embodiments have been described, those skilled in the
art will recognize modifications or variations which might be made
without departing from the present disclosure. The examples
illustrate the various embodiments and are not intended to limit
the present disclosure. Therefore, the description and claims
should be interpreted liberally with only such limitation as is
necessary in view of the pertinent prior art.
Example embodiments are provided so that this disclosure will be
thorough, and will fully convey the scope to those who are skilled
in the art. Numerous specific details are set forth such as
examples of specific components, devices, and methods, to provide a
thorough understanding of embodiments of the present disclosure. It
will be apparent to those skilled in the art that specific details
need not be employed, that example embodiments may be embodied in
many different forms and that neither should be construed to limit
the scope of the disclosure. In some example embodiments,
well-known processes, well-known device structures, and well-known
technologies are not described in detail.
The terminology used herein is for the purpose of describing
particular example embodiments only and is not intended to be
limiting. As used herein, the singular forms "a," "an," and "the"
may be intended to include the plural forms as well, unless the
context clearly indicates otherwise. The terms "comprises,"
"comprising," "including," and "having," are inclusive and
therefore specify the presence of stated features, integers, steps,
operations, elements, and/or components, but do not preclude the
presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof. The
method steps, processes, and operations described herein are not to
be construed as necessarily requiring their performance in the
particular order discussed or illustrated, unless specifically
identified as an order of performance. It is also to be understood
that additional or alternative steps may be employed.
When an element or layer is referred to as being "on," "engaged
to," "connected to," or "coupled to" another element or layer, it
may be directly on, engaged, connected or coupled to the other
element or layer, or intervening elements or layers may be present.
In contrast, when an element is referred to as being "directly on,"
"directly engaged to," "directly connected to," or "directly
coupled to" another element or layer, there may be no intervening
elements or layers present. Other words used to describe the
relationship between elements should be interpreted in a like
fashion (e.g., "between" versus "directly between," "adjacent"
versus "directly adjacent," etc.). As used herein, the term
"and/or" includes any and all combinations of one or more of the
associated listed items.
Although the terms first, second, third, etc. may be used herein to
describe various elements, components, regions, layers and/or
sections, these elements, components, regions, layers and/or
sections should not be limited by these terms. These terms may be
only used to distinguish one element, component, region, layer or
section from another region, layer or section. Terms such as
"first," "second," and other numerical terms when used herein do
not imply a sequence or order unless clearly indicated by the
context. Thus, a first element, component, region, layer or section
discussed below could be termed a second element, component,
region, layer or section without departing from the teachings of
the example embodiments.
Spatially relative terms, such as "inner," "outer," "beneath,"
"below," "lower," "above," "upper," and the like, may be used
herein for ease of description to describe one element or feature's
relationship to another element(s) or feature(s) as illustrated in
the figures. Spatially relative terms may be intended to encompass
different orientations of the device in use or operation in
addition to the orientation depicted in the figures. For example,
if the device in the figures is turned over, elements described as
"below" or "beneath" other elements or features would then be
oriented "above" the other elements or features. Thus, the example
term "below" can encompass both an orientation of above and below.
The device may be otherwise oriented (rotated 90 degrees or at
other orientations) and the spatially relative descriptors used
herein interpreted accordingly.
As used herein, the term "about," when applied to the value for a
parameter of a composition or method of this technology, indicates
that the calculation or the measurement of the value allows some
slight imprecision, resulting (for example) from manufacturing
variability, without having a substantial effect on the chemical or
physical attributes of the composition or method. If, for some
reason, the imprecision provided by "about" is not otherwise
understood in the art with this ordinary meaning, then "about" as
used herein indicates a possible variation of up to 5% in the
value.
* * * * *
References